CONCRETE CONTROL JOINTS

PREVENTING CRACKS IN CONCRETE

Concrete control joints are used to minimize and possibly prevent random cracking in concrete
slabs and floors. Here you will learn how to properly install and where to place
control joints in all types of concrete flatwork to reduce the chances of cracking.

Below is a diagram of a concrete floor showing you the three types of control joints for a concrete floor; isolation joints, contraction joints, and construction joints. For a more detailed explanation, follow the links at the bottom of the page.

Concrete is used in construction for its hardness and very high
compressive strength. Not being a flexible material it doesn't stretch
or bend without cracking. Concrete does move though, it shrinks and
expands. Buildings and other structures also move, contributing to
cracking in slabs and floors.
If the concrete is tied to a structure or itself and tries to move, we
get what's called restraint. This restraint causes tensile forces in the
concrete and in most cases cracking will occur.

Concrete needs to
be able to freely shrink as it dries or to expand and contract with
temperature changes to help minimize any random cracking.

Properly placed control joints will allow concrete to shrink, expand, and contract, reducing the tendency for it to crack.

I've
been in the concrete floor business for over 30 years. I've installed
thousands of lineal feet of control joints in my concrete floors. Most of the time we saw the joints using an early entry soff-cut saw. This method allows us to saw the relief cut very soon after we are done troweling. A big advantage when you're trying to prevent cracks in concrete floors.

I
feel it's very important, if you want to minimize cracking, to take the
time and plan in advance where and what materials you are going to use
to install your control joints.

I still see concrete contractors skip this step and leave customers with cracks in their floors that are ugly and unnecessary.